SANTIAGO, Chile (AP) _ Hundreds of thousands of Chileans chanting ''freedom'' and ''democracy now'' rallied Thursday to demand the end of the 12-year old military regime of Gen. Augusto Pinochet.

The government authorized the rally and police watched from several blocks away. Previous, unapproved rallies here have ended with rioters setting up barricades and throwing stones and gasoline bombs at police armed with clubs and guns. There are often deaths.

The demonstrators, carrying Chilean flags and signs with anti-government slogans, flocked to O'Higgins Park near downtown Santiago, where the rally was held.

''It's the biggest mass rally in Chile's political history,'' said Ricardo Leagos, a leading Socialist.

Organizers of the rally claimed that up to 1 million people jammed the sprawling park, but independent observers estimated the attendance to be about 500,000.

As a police helicopter flew over the park, the noisy crowd started chanting ''Murderers, murderers 3/8''

The rally was called by the Democratic Alliance, a loose coalition of six moderate opposition parties, and was joined by the outlawed leftist coalition called Popular Democratic Movement, which includes the Communist Party.

''He is going to fall, he is going to fall,'' the crowd chanted repeatedly, referring to Pinochet, the 69-year-old army commander who came to power when he overthrew elected Marxist President Salvador Allende.

Allende died in the presidential palace when army troops stormed it in 1973.

Gabriel Valdes, president of the Democratic Alliance and the only speaker at the rally, called Pinochet ''a demagogue'' and said, ''The general is the obstacle to restoration of democracy in Chile.''

Valdes, who also heads the centrist Christian Democratic Party, asked for support for the National Accord for a Transition to Democracy, a Catholic Church-sponsored plan signed by 11 moderate political groups.

The proposal, which has been rejected by Pinochet, calls for an early congressional election that could lead to reform of Pinochet's 1980 constitution.

The constitution extends Pinochet's term until at least 1987, and possibly until 1997.

''It we don't support the national accord, we are heading toward civil war,'' warned Valdes, a former undersecretary of the United Nations.

Earlier, police reported that at least eight bombs exploded Wednesday night in various places. One one wounded three youngsters at a shelter for homeless children.